In a satellite positioning system using a satellite positioning receiver of GNSS type arranged on board a terrestrial, maritime or airborne vehicle, the data signals enabling the receiver to calculate its positioning originate from different satellites belonging to a constellation of positioning satellites. The constellation comprises at least four satellites for determining four unknowns corresponding to the geographic coordinates x, y, z and temporal coordinates t of the receiver. The positioning of the vehicle by the receiver is performed in two steps. In a first step, the receiver acquires radiofrequency signals constituting navigation signals originating from the four satellites of the constellation and in a second step the receiver evaluates the distances separating the vehicle from the four satellites from which the signals have been received and determines the position of the vehicle by using a trilateration method.
An error committed on the position of a vehicle can have disastrous consequences in an application relating to civil aviation geolocated road toll.
There are many sources of positioning error that can affect the validity of position information determined by a satellite positioning system. A positioning error can be due to a technical problem on the reception of the GNSS signals, such as, for example, a failure of the receiver, or a failure of the information transmitted by the constellation of satellites, or a satellite failure. The reliability of the position determined by a satellite positioning system depends also on the environment in which the vehicle is located and a positioning error can also be due notably to a stray reflection on a building, or to interference on the signal.
In the case of an aeronautical application, the receiver is not constrained by any obstacle, so that the radiofrequency signals are received directly from the satellites, without reflection on any wall. In this case, there are SBAS (Satellite-Based Augmentation Systems) systems that make it possible to provide confidence information relating to the position calculated by the receiver of an aeronautical vehicle. The SBAS systems permanently monitor and limit the errors committed on the orbit of the satellites, on the synchronization of each satellite with the time reference of the constellations and the errors induced by the propagation of the radiofrequency signals at high atmosphere and in particular in the ionosphere. The information supplied by an SBAS system enables the receiver of the aeronautical vehicle to supply the position of the vehicle as well as a position error limit.
In the case of an aeronautical application, it is also known practice to use an INS/GNSS (Inertial Navigation System/Global Navigation Satellite System) hybrid equipment item combining the information supplied by an inertial unit and the measurements supplied by the satellite navigation system including a GNSS receiver, to obtain vehicle position and speed information. The INS/GNSS hybridization architectures can use different types of coupling between a GNSS receiver and an inertial unit. The coupling can be done either from the calculated position of the GNSS receiver, or from rough measurements of the frequency or the pseudo-distances determined from the navigation signals received from the satellites, or from even more elementary information calculated in the receiver, this latter type of coupling being called ultra-tight coupling. The inertial unit supplies information with little noise and that is accurate in the short term, but the accuracy of the measurements is degraded over the long term because of the drifts of the inertial sensors. The accuracy of the measurements supplied by the GNSS receiver makes it possible to control the inertial drift, and the inertial measurements make it possible to filter the noise on the measurements of the GNSS receiver. This equipment also calculates protection radii around the calculated position which make it possible to contain the position error at a given integrity risk. The protection radii can be calculated by using a channel filter, for example a Kalman filter, which comprises a model of the behaviour of the GNSS receiver and supplies an estimation of receiver distance and speed information. A parameter, called innovation, corresponding to the difference between the measurement of the distance information supplied by the satellite and the estimation of this distance information supplied at the output of the channel filter is then calculated. When the behaviour of the receiver corresponds to the model included in the filter, the innovation parameter has a value close to zero. Otherwise, the GNSS measurement is errored. The innovation parameter therefore makes it possible, in the case of an aeronautical application, to identify GNSS measurements affected by wide errors, possibly occurring notably when a satellite has failed.
The geolocated road toll applications consist in determining the route taken by a terrestrial vehicle provided with a GNSS receiver and in billing a user of the terrestrial vehicle when the route taken is subject to a toll. Since the billing is dependent on the road used, the receiver must deliver two complementary information items concerning, on the one hand, the position of the vehicle and, on the other hand, the trajectory of the vehicle. Since this information gives rise to a billing, it is also necessary to determine trust information concerning the trajectory used.
The integrity of a GNSS position in a constrained medium, for example urban, woody area, mountainous area, is difficult to characterize, notably because of the imprecision of the modeling of the local propagation phenomena. The identification and the characterization of the quality of the GNSS measurements produced by a receiver is all the more difficult. Currently, the checking of the integrity of a GNSS position of a terrestrial vehicle is done in the same way as for civil aviation. When the navigation is performed in an unconstrained environment, for example in the countryside or in a fairly scattered town, this method is effective. However, in the case of navigation in a constrained environment, the conditions of reception of the radiofrequency signals are much more complex and much less controlled than in the case of an aeronautical application and the signals received are much more noisy and have a much weaker intensity. The error models designed for the applications of civil aviation therefore do not correspond to the constrained environments and it is not possible to clearly identify the position of a terrestrial vehicle on a traffic lane. Moreover, for a constrained environment, no reliable GNSS measurement quality indicator is currently available.